Inherits: If T is a (possibly cv-qualified)
union type then inherits from true_type,
otherwise inherits from false_type.
Currently requires some kind of compiler support, otherwise unions are identified
as classes.

C++ Standard Reference: 3.9.2 and 9.5.

Compiler Compatibility: Without (some as
yet unspecified) help from the compiler, we cannot distinguish between union
and class types using only standard C++, as a result this type will never
inherit from true_type,
unless the user explicitly specializes the template for their user-defined
union types, or unless the compiler supplies some unspecified intrinsic that
implements this functionality. Currently (May 2005) only Visual C++ 8 has
the necessary compiler intrinsics
to make this trait "just work" without user intervention.

Header:#include<boost/type_traits/is_union.hpp>
or #include<boost/type_traits.hpp>